This invention relates to articles prepared from ethylene interpolymers made by an interpolymerization process. The processes utilize at least one homogeneous polymerization catalyst and at least one heterogeneous polymerization catalyst in separate reactors connected in series or in parallel. Interpolymers produced from such processes are thermoplastic and have surprisingly beneficial properties, including improved room and low temperature impact and tear properties, high modulus and higher crystallization temperatures, while maintaining equivalent or improved processability as compared to the individual blend components. The resins of the present invention are useful in making molded or shaped articles, film, and the like.
There are known several polymerization processes for producing polyethylene and ethylene interpolymers, including suspension, gas-phase and solution processes. Of these, the solution process is of commercial significance due to the advantages described in U.S. Pat. No. 4,330,646 (Sakurai et al.), the disclosure of which is incorporated herein by reference. A most advantageous solution process would be found if the temperature of the polymerization solution could be increased and the properties of the polymers suitably controlled. U.S. Pat. No. 4,314,912 (Lowery et al.), the disclosure of which is incorporated herein by reference, describes a Ziegler-type catalyst suitable for use in high temperature solution polymerization processes. U.S. Pat. No. 4,612,300 (Coleman, III), the disclosure of which is incorporated herein by reference, and U.S. Pat. No. 4,330,646 describe a catalyst and solution polymerization process for producing polyethylenes having a narrow molecular weight distribution. U.S. Pat. No. 4,330,646 also describes a process for producing polyethylenes with a broader molecular weight distribution in a solution process. These processes are based on heterogeneous Ziegler type catalysts, which produce interpolymers with broad composition distributions regardless of their molecular weight distribution. Such ethylene polymers have deficiencies in some properties, for instance, poor transparency and poor anti-blocking properties.
Solution polymerization processes for producing ethylene interpolymers with narrow composition distributions are also known. U.S. Pat. No. 4,668,752 (Tominari et al.), the disclosure of which is incorporated herein by reference, describes the production of heterogeneous ethylene copolymers with characteristics which include a narrower composition distribution than conventional heterogeneous copolymers. The utility of such polymer compositions in improving mechanical, optical and other important properties of formed or molded objects is also described. The complex structures of the copolymers necessary to achieve such advantages are finely and difficultly controlled by nuances of catalyst composition and preparation; any drift in which would cause a significant loss in the desired properties. U.S. Pat No. 3,645,992 (Elston), the disclosure of which is incorporated herein by reference, describes the preparation of homogeneous polymers and interpolymers of ethylene in a solution process operated at temperatures of less than 100xc2x0 C. These polymers exhibit a xe2x80x9cnarrow composition distributionxe2x80x9d, a term defined by a comonomer distribution that, within a given polymer molecule, and between substantially all molecules of the copolymer, is the same. The advantages of such copolymers in improving optical and mechanical properties of objects formed from them is described. These copolymers, however, have relatively low melting points and poor thermal resistance.
U.S. Pat. No. 4,701,432 (Welborn, Jr.), the disclosure of which is incorporated herein by reference, describes a catalyst composition for the production of ethylene polymers having a varied range of composition distributions and/or molecular weight distributions. Such compositions contain a metallocene and a non-metallocene transition metal compound supported catalyst and an aluminoxane. U.S. Pat. No. 4,659,685 (Coleman, III et al.), the disclosure of which is incorporated herein by reference, describes catalysts which are composed of two supported catalysts (one a metallocene complex supported catalyst and the second a non-metallocene transition metal compound supported catalyst) and an aluminoxane. The disadvantages of such catalysts in the commercial manufacture of ethylene polymers are primarily twofold. Although, the choice of the metallocene and a non-metallocene transition metal compounds and their ratio would lead to polymers of controlled molecular structure, the broad range of ethylene polymer structures required to meet all the commercial demands of this polymer family would require a plethora of catalyst compositions and formulations. In particular, the catalyst compositions containing aluminoxanes (which are generally required in high amounts with respect to the transition metal) are unsuitable for higher temperature solution processes as such amount of the aluminum compounds result in low catalyst efficiencies and yield ethylene polymers with low molecular weights and broad molecular weight distributions.
Thus, it would be desirable to provide an economical solution process, which would provide ethylene interpolymers with controlled composition and molecular weight distributions. It would be additionally desirable to provide a process for preparing such interpolymers with reduced complexity and greater flexibility in producing a full range of interpolymer compositions in a controllable fashion.
Useful articles which could be made from such interpolymer compositions include films (e.g., cast film, blown film or extrusion coated types of film), fibers (e.g., staple fibers, melt blown fibers or spunbonded fibers (using, e.g., systems as disclosed in U.S. Pat. No. 4,340,563, U.S. Pat. No. 4,663,220, U.S. Pat. No. 4,668,566, or U.S. Pat. No. 4,322,027, all of which are incorporated herein by reference), and gel spun fibers (e.g., the system disclosed in U.S. Pat. No. 4,413,110, incorporated herein by reference)), both woven and nonwoven fabrics (e.g., spunlaced fabrics disclosed in U.S. Pat. No. 3,485,706, incorporated herein by reference) or structures made from such fibers (including, e.g., blends of these fibers with other fibers, e.g., PET or cotton)), and molded articles (e.g., blow molded articles, injection molded articles and rotational molded articles).
Rotational molding (also known as rotomolding), is used to manufacture hollow objects from thermoplastics. In the basic process of rotational molding, pulverized polymer is placed in a mold. While the mold is being rotated, the mold is heated and then cooled. The mold can be rotated uniaxially or biaxially and is usually rotated biaxially, i.e., rotated about two perpendicular axes simultaneously. The mold is typically heated externally and then cooled while being rotated. As such. rotomolding is a zero shear process and involves the tumbling, heating and melting of thermoplastic powder, followed by coalescence, fusion or sintering and cooling. In this manner, articles may be obtained which are complicated, large in size, and uniform in wall thickness.
Many compositions have been employed in rotational molding. For example, U.S. Pat. No. 4,857,257 teaches rotational molding compositions comprising polyethylene, peroxide cross-linker, and a metal cationic compound while U.S. Pat. No. 4,587,318 teaches crosslinked compositions comprising ethylene terpolymer and organic peroxide.
Research disclosure, RD-362010-A describes blends of traditionally catalyzed polyolefins, especially very low or ultralow density polyethylenes with densities of 0.89 to 0.915 g/cm3 with polyolefins made using single-site, metallocene catalysts. These blends are especially suited to rotational molding providing good control over the balance of processability and improved environmental stress crack resisitance (ESCR) and tear properties.
In the case of rotational molding, the final density and melt index of the compositions is typically a compromise between processability and end-product properties. Conventional knowledge teaches that increasing polymer density (or modulus) results in decreasing impact, and increasing melt index (or decreasing molecular weight) results in increased processability and corresponding decreases in ESCR and impact. Furthermore, increased branching has been known to result in inferior processability. As a result, one typically must choose which property to increase with the expectation that the other property must be decreased.
Thus it would be highly desirable to prepare molding compositions with improved processability (even when the zero or low shear viscosity or branching is increased) and improved room and low temperature impact and tear properties, improved optical properties, high modulus and higher thermal stability""s, without necessarily decreasing the polymer density. Such improvements would be advantageous in a wide range of applications, including but not limited to molding and especially rotational molding., films, fibers and foams.
We have now discovered fabricated articles prepared by a polymerization processes for preparing interpolymer compositions of controlled composition and molecular weight distributions. The processes utilize at least one homogeneous polymerization catalyst and at least one heterogeneous polymerization catalyst in separate reactors connected in series or in parallel.
The First Process comprises the steps of:
1. A process for preparing an ethylene/xcex1-olefin interpolymer composition, comprising the steps of:
(A) reacting by contacting ethylene and at least one other xcex1-olefin under solution polymerization conditions in the presence of a homogeneous catalyst composition containing either no aluminum cocatalyst or only a small amount of aluminum cocatalyst in at least one reactor to produce a solution of a first interpolymer which has a narrow composition distribution and a narrow molecular weight distribution,
(B) reacting by contacting ethylene and at least one other xcex1-olefin under solution polymerization conditions and at a higher polymerization reaction temperature than used in step (A) in the presence of a heterogeneous Ziegler catalyst in at least one other reactor to produce a solution of a second interpolymer which has a broad composition distribution and a broad molecular weight distribution, and
(C) combining the solution of the first interpolymer with the solution of the second interpolymer to form a high temperature polymer solution comprising the ethylene/xcex1-olefin interpolymer composition, and
(D) removing the solvent from the polymer solution of step (C) and recovering the ethylene/xcex1-olefin interpolymer composition.
These polymerizations are generally carried out under solution conditions to facilitate the intimate mixing of the two polymer-containing streams. The homogeneous catalyst is chosen from those metallocene-type catalysts, which are capable of producing ethylene/xcex1-olefin interpolymers of sufficiently high molecular weight under solution process polymerization conditions (e.g., temperatures greater than or equal to about 100xc2x0 C.). The heterogeneous catalyst is also chosen from those catalysts, which are capable of efficiently producing the polymers under high temperature (e.g., temperatures greater than or equal to about 180xc2x0 C.) solution process conditions.
In addition, there is provided a second process for preparing interpolymer compositions of controlled composition and controlled molecular weight distributions.
The Second Process comprises the steps of:
A process for preparing an ethylene/xcex1-olefin interpolymer composition, comprising the steps of:
(A) polymerizing ethylene and at least one other xcex1-olefin in a solution process under suitable solution polymerization temperatures and pressures in at least one reactor containing a homogeneous catalyst composition containing either no aluminum cocatalyst or only a small amount of aluminum cocatalyst to produce a first interpolymer solution comprising a first interpolymer having has a narrow composition distribution and a narrow molecular weight distribution, and
(B) sequentially passing the interpolymer solution of (A) into at least one other reactor containing a heterogeneous Ziegler catalyst, ethylene and at least one other xcex1-olefin under solution polymerization conditions and at a polymerization temperature higher than that used in (A), to form a high temperature polymer solution comprising the ethylene/xcex1-olefin interpolymer composition, and
(C) removing the solvent from the polymer solution of step (B) and recovering the ethylene/xcex1-olefin interpolymer composition.
In either process, the homogeneous catalyst composition preferably exhibits a high reactivity ratio and very readily incorporates higher xcex1-olefins.
The homogeneous catalysts employed in the production of the homogeneous ethylene interpolymer are desirably derived from monocyclopentadienyl complexes of the Group IV transition metals, which contain a pendant bridging group, attached to the cyclopentadienyl ring which acts as a bident ligand. Complex derivatives of titanium in the +3 or +4 oxidation state are preferred.
In another aspect of this invention, there are provided novel interpolymers of ethylene and at least one xcex1-olefin, wherein the interpolymers have controlled composition and molecular weight distributions. The interpolymers have improved mechanical, thermal and optical properties and, surprisingly, the polymer compositions obtained by the processes described herein provide superior properties to materials obtained by merely blending the solid polymers obtained from process step (A) or (B) individually, in the First Process listed above.
The novel polymer compositions of the present invention can be ethylene or C3-C20 xcex1-olefin homopolymers, preferably propylene or, more preferably, interpolymers of ethylene with at least one C3-C20 xcex1-olefin and/or C4-C18 diolefins. Interpolymers of ethylene and 1-octene are especially preferred. The term xe2x80x9cinterpolymerxe2x80x9d is used herein to indicate a copolymer, or a terpolymer, or the like. That is, at least one other comonomer is polymerized with ethylene to make the interpolymer.
In another aspect of the invention, thermoplastic compositions have been discovered which are especially suitable for rotational and injection molding and have improved physical and/or mechanical properties. In many cases, processability is also improved during rotational molding, as reflected in, for example, shorter cycle times, faster sintering, and/or the ability to fabricate articles over wide ranges of processing temperatures. For injection molding, the compositions may also exhibit shorter cycle times due to decreased set up times.
Advantageously, the compositions often exhibit one or more of the following: improved low temperature and/or room temperature impact, improved environmental stress crack resistance, and acceptable flexural and secant modulus, increased upper service temperature.
The compositions of the present invention with improved impact properties can also be utilized in other fabrication processes including, but not limited to blow molding, calendaring, pulltrusion, cast film, and blown film.